1. Technical Field
The present invention relates to a satellite signal receiving device, an electronic timepiece, and a satellite signal receiving method for receiving satellite signals transmitted from positioning information satellites such as GPS satellites and Quasi-Zenith Satellites.
2. Related Art
Operation of the Quasi-Zenith Satellite System (QZSS) has started in Japan to augment the Global Positioning System (GPS) that uses GPS satellites. Because the quasi-zenith satellites (QZS) used in the Quasi-Zenith Satellite System transmit signals compatible with the L1 C/A signals used by the GPS system, a quasi-zenith satellite can also be used as a GPS satellite.
The Quasi-Zenith Satellite System is a satellite system that uses a combination the quasi-zenith satellites on plural orbits (quasi-zenith orbits) so that one satellite is always near the zenith over Japan. The path of the quasi-zenith satellites orbits the Earth on the same period as the rotation of the Earth on a specific inclination (the inclination of the orbital plane to the equatorial plane). As a result, the satellites follow an asymmetrical figure-8 pattern as shown in FIG. 16.
Because a quasi-zenith satellite is always near the zenith in Japan, the possibility of being able to receive a signal from a quasi-zenith satellite is high even in locations where GPS satellites are hidden by mountains or buildings.
Because a quasi-zenith satellite can only remain at a high latitude position for part of its orbital period, three or more satellites are planned to be placed in orbit on the same path so that one quasi-zenith satellite is always stationed above Japan. A Quasi-Zenith Satellite System of four satellites will be deployed with three satellites on a quasi-zenith orbit and one satellite on a geostationary orbit. As a result, at least one quasi-zenith satellite will always be located above Japan, and the probability of successful reception should be improved.
To shorten the time required for positioning in a cold start mode, which occurs when operation starts with no satellite orbit information stored locally, JP-A-2006-317225 discloses a satellite signal receiver that receives signals transmitted from a quasi-zenith satellite described above.
The satellite signal receiver disclosed in JP-A-2006-317225 includes a quasi-zenith satellite search unit that locates a quasi-zenith satellite, and a non-quasi-zenith satellite that locates a non-quasi-zenith satellite after the quasi-zenith satellite search unit finds a quasi-zenith satellite.
However, with the satellite signal receiver (satellite signal receiving device) described in JP-A-2006-317225, the search for a quasi-zenith satellite always occurs before the search for a non-quasi-zenith satellite (such as a GPS satellite). More specifically, JP-A-2006-317225 assumes that reception will only occur in locations where a quasi-zenith satellite is stationed directly overhead, such as by a car navigation system for automobiles used in Japan. As a result, JP-A-2006-317225 does not anticipate use in devices such as wristwatches, for example, that may move with the user when travelling to a country where quasi-zenith satellites do not exist (are not used).
Therefore, if the satellite signal receiver described in JP-A-2006-317225 is used in a country where quasi-zenith satellite signals cannot be received, reception from non-quasi-zenith satellites will be delayed because quasi-zenith satellites that cannot be received are searched for first, the satellite search time becomes long, and reception time increases significantly.
This problem is not limited to satellite signal receivers that can receive signals from the quasi-zenith satellites used in Japan's Quasi-Zenith Satellite System (QZSS), and is shared by other satellite signal receiving devices that can receive signals from GNSS satellites used in the Global Navigation Satellite System (GNSS) similar to the GPS system, and RNSS satellites used in Regional Navigation Satellite Systems (RNSS), which can only be received in limited areas similarly to QZSS.